Multiple grid harmonic generator



April 4, 1939. M. G'. CROSBY l' 2,152,759

MULTIPLE GRID HARMONIC GENERATOR I Filed DGO. l0, '1936 v VAVAVAVA\ v n soRcEoF l' I I l ALTER/MTI scum/ENTv C +39 +B Patented Apr. 4, 1939 UNITED STATES PATENT OFFICE Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 10, 1936, Serial No. 115,129

11 Claims.

This invention discloses a form of harmonic generator or frequency multiplier wherein the voltage to be multiplied is fed to two grids of a multiple-grid tube and use is made of the fact 5 that the overall operation of the tube is a power function curve whose exponent is the product of the exponents of the power functions o-f the individual grids. Thus, with both grids operating 1 on the square-law portion of theircharacteristics, the resultant characteristic would have a fourth-power law. In this way, a high order of harmonic generation may be effected with an economy of apparatus. Less voltage will be required for a given order of generation in the tube than would be required if only a single grid were utilized.

In the prior art, frequency multiplication is generally accomplished by heavily loading a class C amplifier so that a poor output wave-form is 20 produced which is rich in harmonics. Then by suitable selectivity, the desired harmonic is chosen. By using such an amplifier fed in push-pull with the output connected in push-push, even harmonics are produced and odd harmonics cancelled. The ordinary push-pull connection can- 25 cels the even and produces the odd harmonics.

The harmonic generator of this invention may be connected in the same manner as the prior art generators, but instead of feeding the signal to a single grid, it is fed to twoigrids. The theory of operation of the pentode tube having two grids excited for the production of harmonics is an additional development resulting from my study of pentode detectors and their actions as described in Crosby United States applications 35 #716,469 sied March 2o, 1934, Patent #2,063,588 dated December 8, 1936; #25,026 filed June 5, 1935; and #25,231 led June 6, 1935, Patent #2,087,429 dated July 20, 1937.

As I look at this tube, the essential characteristic which allows its use for these detectors and the harmonic generator is the fact that the mutual conductance of one grid is varied (linearly in the case of the detectors) by the Voltage on the other grid. In order to form this characteristie, it is necessary to form a virtual cathode by means of the screen-grid. Hence, the suppressor, or second grid, operates on the electrons emitted by this virtual cathode in the manner of a space-charge grid tube. If this virtual cathode were not present, the second grid would not change the slope of the first grid characteristic,

but would only translate it to a parallel position 55 of the same slope. ASuch a case would be presented by a tube commercially known as the RCA-59 Where the control grid consists of two separate grids tied together.

This essential characteristic can, of course, be

50 produced in a screen-grid tube in the manner of (c1. 25o-3s) screen-grid modulation and in a triode in the manne-r of plate modulation. However, the two isolated grids of the pentode are obviously the moet convenient arrangement.

For this harmonic generator, this particular characteristic is taken advantage of by feeding both grids so as to utilize the overall or resultant characteristic of both grids taken together. In this manner, the two grids which are linear taken separately are square-law taken together, since, While each grid is varying the' plate current due to its voltage variation, it is also varying the mutual conductance of the other grid. Consequently, the resultant effect is a rapid variation of plate current withgrid voltages which turns out to be square-law for theY both-grids-linear case.

l With the tube adjusted for square-law in this manner, it could also be used in the place of an ordinary square-law detector by feeding the signal to both grids. However, except in the case where a very pure square-law characteristic were desired, the ordinary square-law tube would probably be most practical.

In describing my invention in detail reference will be made to the accompanying drawing where- 1n Figures 1, 2, 3, show three different modifications of my harmonic generator circuit wherein two grids of a pentode are excited by a fundamental frequency and a harmonic frequency is derived from the output circuit; and

Figures 4 and 5 are tube characteristic curves illustrating the grid voltage plotted against plate current for different iixed potentials on one grid and variable potentials on the other grid.

Referring to the drawing and in particular to Figure 1, any wave energy or alternating voltage from any source is fed by way of a transformer secondary l directly to the suppressor grid l2 and by way of a coupling condenser 2 to the control grid l of an electron discharge device li of the pentode type. Biasing potentials are applied as shown, by Way of resistance 3 and secondary I to the electrodes l and l2 respectively.-

Since the two grids may use diiferent biasing potentials the coupling condenser 2, which serves also as a blocking condenser, permits the control gridA I to be biased separately by way' of resistor 3. The anode I 4 is connected to an output circuit 5. The harmonic output is selected from the plate circuit of the tube by means of tuned circuit 5. 5 might also be a band pass or high or low pass iilter.

The reason that such a multi-grid connection is more suitable for harmonic generation is shown as follows:-The process of harmonic generation depends for its operation upon the fact that the tube operates non-linearly. Thus, a square-law or second-power law tube produces second harmonies; a cube-law tube, third harmonics, and so on. Hence, if the voltage e=E sin wt (1) is passed through a pure square-law tube, the output is which output consists of a direct current component,

and a second harmonic component,

2 EEZ-cos 2Wt The characteristics of the multi-grid tube of the type described in Crosby United States applications #716,469 led March 20, 1934, Patent #2,- 063,588 dated December 8, 1936; and #25,231 led June 6, 1935, Patent #2,087,429 dated July 20, '1937 are shown in Figures 4 and 5 of the latter application. These tube characteristics are also shown in Figures 4 and 5 of the present disclosure. In these figures, the plate current Versus grid bias of grids ill and l2 are plotted for different biasing potentials applied thereto. From these characteristics it can be seen that the variable output due to each individual grid is given ear terms will produce an overall square-law effect and the product of the two square-law terms will produce a fourth-power eiect and so on. In this manner, the higher degree terms of the output are made stronger so that the tube is a more effective harmonic generator. That this is true may be shown by multiplying (4) out and substituting the voltage to be multiplied for ei and e2. Equation (4) multiplied out gives From (5) it can be seen that the term which was (arci) for a single grid is now (aimerez). Consequently, if e1=e2=E sin wt, the output would be a1aqE2 sin2 wt= instead of (a1E sin wt) which would be the output for a single grid. Hence, by applying the signal to both grids, the output term has doubled in frequency. In this manner, all the other terms tend to produce a more multiplied output also.

By applying the voltage to be multiplied in push-pull according to Figure 2, the fundamental and odd harmonics tend to cancel in the same manner as the above mentioned multipliers of the prior art. This can be seen from an examination of (5). Since e1 will be negative with respect to ez, term (lc2Ea1e1) will tend to buck term (k1Ea2e2) likewise, term (k2E"c1e13) will tend to buck term 7c1Ec2e23) etc. These terms are those producing fundamental and odd harmonies.

The circuit of Figure 2 is essentially the same as the one of Figure 1. The only difference lies in the push-pull input circuit.

Other well known features as are applied to the prior art frequency multipliers are applicable to this type of multiplier. For instance, grid leak and condenser bias may be applied in the manner shown in Figure 3. The push-pull circuit of Figure 2 also lends itself to this kind of bias. n

The essential feature of this type of multi-grid harmonic generator lies in the fact that the Voltage on one grid varies the mutual conductance of the other grid. In the tube described in this disclosure, this characteristic is obtained by virtue of the fact that the suppressorY grid operates on the electrons emitted from the virtual cathode formed by the screen grid operating as spacecharge grid. Consequently, there are, virtually, two tubes in series and one gridY operates to change the slope of the characteristic of the other. Hence, it can be seen that if a tube were constructed with a second space-charge grid 'and a third control grid, the principle of this invention could be carried still further. That is, lthis triple-control-.grid double-space-charge-grid tube would make an overall cube-law characteristic out of the linear characteristics of each grid, and so on. In this way, the number of grids employed may be carried to any extent which is limited only by the practical considerations in the construction of the tube.

Any tube may be used to obtain this desired characteristic Whereinfone grid has its mutual conductance dependent upon the other grid voltage. Thus, a four element tube might be used by using the control-grid for one of the grids and the screen-grid for the other in the manner of screen-grid modulation. A triode could also be used in the manner of plate modulation. In fact, any kind of tube capable of being modulated could be used.

It will be apparent that various combinations may be effected which will favor the production of certain harmonics. Thus, one grid might be adjusted linear and the other square-law to favor the third harmonic or one square-law and the other cube-law to favor the fifth harmonic.

I claim:

1. In a system for producing harmonically related alternating potentials by means of alternating potentials of fundamental frequency, an electron discharge device having a. control grid, 'a cathode, an anode, and a suppressor grid, means for impressing alternating current of the funda'- mental frequency on said control grid and said suppressor grid, means for producing a virtual cathode effect in said tube in the region between said control grid and said suppressor grid, a circuit tuned to a harmonic of the fundamental frequency connected between said anode and cathode, and means for biasing said control grid and said suppressor grid relative to said cathode.

2. In a system for producing harmonically rea control grid, a cathode, an anode, a suppressor grid, and means for producing -a virtual cathode in the space between said supressor grid and control grid, means for impressing alternating cur- ,rents of the fundamental frequency in phase op- )position on said control grid and said suppressor grid, a circuit tuned to an even harmonic of the fundamental frequency connected between said anode and cathode, and means for biasing said 'control grid and said suppressor grid relative to said cathode.

3. In a system for producing harmonically related potentials by means of potentials of a fundamental frequency, an electron discharge device having a control grid, a cathode, an anode, and a suppressor grid, means for impressing potentials of the fundamental frequency on said control grid and said suppressor grid, a circuit tuned to a harmonic of the fundamental frequency connected between said anode and cathode, means for biasing said control grid and said suppressor grid relative to said cathode and an auxiliary electrode in said suppressor grid tube maintained at a potential such that it serves to produce a virtual cathode in space between said suppressor grid and control grid.

4. In a system for producing harmonically related frequencies by means of alternating current potentials, an electron discharge device having a cathode, a co-ntrol grid, an anode, a shielding electrode between said control grid and anode, 'and an auxiliary electrode between said shielding electrode and anode, means for biasing said control grid and said auxiliary electrode relative to said cathode, means for maintaining said shielding electrode positive relative to said cathode an amount sufficient to produce a virtual cathode in the i'leld of said shielding electrode, a circuit tuned to a harmonic of said fundamental frequency connected between said anode and cathode and means for applying potentials of the fundamental frequency to said control electrode and said auxiliary electrode.

5. In a system for producing higher and harmonically related frequencies by means of alternating current potentials, an electron discharge device having a cathode, an anode, and a plurality of control electrodes, a circuit tuned to a harmonic of said fundamental frequency connected between said anode and cathode means for applying potentials of the fundamental frequency to said control electrodes, means for biasing said control electrodes relative to said cathode by potentials such that the applied potentials on one control electrode alters the slope of the control electrode potential anode current characteristic of the other control electrode, and means for producing a virtual cathode effect in the space between said control electrodes.

6. In a frequency multiplying system, an electron discharge device having an anode, a cathode, a control grid, a shielding electrode between said control grid and anode, and a suppressor electrode between said shielding electrode and anode, an alternating current circuit connected between said anode and cathode, means for impressing a positive potential on said shielding electrode to produce in the field thereof a virtual cathode, means for biasing said control grid and said suppressor electrode negative relative to said cathode, and means for impressing alternating voltages to be multiplied on said control grid and on said suppressor electrode, whereby the conductance of said control grid is controlled by the po- ,tential applied to said suppressor electrode.

7. In a frequency multiplying system, an electron discharge device having an anode, a cathode, a control grid, a shielding electrode between said control grid and anode, and a suppressor electrode between said shielding electrode and anode, an alternating current circuit connected between said anode and cathode, means for impressing a positive potential on said shielding electrode to produce in the field thereof a virtual cathode, means for biasing said control grid and said suppressor electrode negative relative to said cathode, means for impressing alternating voltages to be multiplied on said suppressor electrode, and means for impressing said voltages to be multiplied on said control grid to thereby vary the conductance of said suppressor electrode.

8. In a frequency multiplying system, an electron discharge device having an anode, a cathode, a control grid, a shielding electrode between said control grid and anode, and an auxiliary electrode between said shielding electrode and anode, an alternating current circuit connected between said anode and cathode, means for impressing a positive potential on said shielding electrode to produce in the field thereof a virtual cathode, means for biasing said control grid and said auxiliary electrode negative relative to said cathode, means for impressing alternating voltages to be multiplied in phase on said control grid and auxiliary electrode whereby the conductance of said auxiliary electrode and of said control grid is controlled by voltages applied to the other.

9. A system as recited in claim 6 wherein said alternating current voltages are applied in phase opposition to said control grid and suppressor electrode.

l0. In a harmonic generator, an electron discharge tube having a cathode, an output electrode, and a plurality of controlling electrodes, means for biasing a plurality of said controlling electrodes relative to said cathode by potentials such that each controlling electrode has a square law controlling electrode voltage plate current characteristic, means for producing a virtual cathode effect in the region between two of said controlling electrodes so biased, means for applying wave energy of a fundamental frequency to each of said controlling electrodes so biased, and means for deriving energy from said output electrode the frequency of which is equal to 2n, where n is the number of the controlling electrodes so biased.

11. In a frequency multiplying system, an electron discharge tube having an anode, a cathode electrode, and a plurality of control electrodes, means for producing a virtual cathode in the space between said control electrodes, means for biasing said control electrodes relative to said cathode electrode by potentials such that if signal voltages were applied to each control electrode the respective individual control of said control electrodes would produce anode current expressible as a power series of the applied signal voltage, means for applying signal voltages to be multiplied to a plurality of said control electrodes simultaneously whereby the anode current is expressible as a power series which varies as the product of the aforementioned power series, and an output circuit connected with said anode and tuned to a harmonic of the applied Voltage frequency of the same order as a term of said last named power series.

MURRAY G. CROSBY. 

